大约有 47,000 项符合查询结果（耗时：0.0342秒） [XML]
how do I query sql for a latest record date for each user
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select t.username, t.date, t.value
from MyTable t
inner join (
select username, max(date) as MaxDate
from MyTable
group by username
) tm on t.username = tm.username and t.date = tm.MaxDate
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SQL: How to properly check if a record exists
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It's better to use either of the following:
-- Method 1.
SELECT 1
FROM table_name
WHERE unique_key = value;
-- Method 2.
SELECT COUNT(1)
FROM table_name
WHERE unique_key = value;
The first alternative should give you no result or one result, the second count should be zero or on...
Difference between Select and ConvertAll in C#
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Select is a LINQ extension method and works on all IEnumerable<T> objects whereas ConvertAll is implemented only by List<T>. The ConvertAll method exists since .NET 2.0 whereas LINQ was introduced with 3.5.
You s...
Selecting element by data attribute
Is there an easy and straight-forward method to select elements based on their data attribute? For example, select all anchors that has data attribute named customerID which has value of 22 .
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Finding duplicate values in a SQL table
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SELECT
name, email, COUNT(*)
FROM
users
GROUP BY
name, email
HAVING
COUNT(*) > 1
Simply group on both of the columns.
Note: the older ANSI standard is to have all non-aggregated columns in the GROUP BY ...
How do you connect to multiple MySQL databases on a single webpage?
...d);
$dbh2 = mysql_connect($hostname, $username, $password, true);
mysql_select_db('database1', $dbh1);
mysql_select_db('database2', $dbh2);
Then to query database 1 pass the first link identifier:
mysql_query('select * from tablename', $dbh1);
and for database 2 pass the second:
mysql_query...
Find rows that have the same value on a column in MySQL
...sses and how many times they're used, with the most used addresses first.
SELECT email,
count(*) AS c
FROM TABLE
GROUP BY email
HAVING c > 1
ORDER BY c DESC
If you want the full rows:
select * from table where email in (
select email from table
group by email having count(*) &g...
How do I check if a column is empty or null in MySQL?
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This will select all rows where some_col is NULL or '' (empty string)
SELECT * FROM table WHERE some_col IS NULL OR some_col = '';
share
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SSH to Elastic Beanstalk instance
... region.
Configure Security Group
In the AWS console, open the EC2 tab.
Select the relevant region and click on Security Group.
You should have an elasticbeanstalk-default security group if you have launched an Elastic Beanstalk instance in that region.
Edit the security group to add a rule for S...
SQL Server loop - how do I loop through a set of records
how do I loop through a set of records from a select?
8 Answers
8
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